Such weft thread brakes are used in shuttleless looms for enabling the weft thread insertion devices, such as gripper rods, to select the intended weft thread out of a plurality of weft threads while the selected weft thread is held in a tensioned or stretched condition. The weft thread insertion members, for example the grippers, which are inserted into and withdrawn from respective opposite sides of the loom shed, are subject to substantial accelerations and decelerations during the weft thread insertion. After a presented weft thread has been seized by an insertion gripper, the thread is transported very rapidly to the center of the loom shed. As a result, the thread is rapidly reeled off the supply spool. At the center of the loom shed, the weft thread is transferred from the inserting gripper to the withdrawing gripper, which has moved into the loom shed from the opposite side. During the transfer, the just mentioned rapid thread insertion is briefly interrupted. However, directly following the transfer, the thread, now held by the withdrawal gripper, is again pulled at a high speed completely through the loom shed. Once the thread has been pulled completely through the loom shed, the thread is released by the gripper at the end of the insertion path, whereby the thread is stopped. In this type of thread insertion, it is important that the weft thread remains taut during the entire time of thread insertion, even, for example, during the brief interruption of the thread insertion operation during the thread transfer from one gripper to the other and when the thread is stopped once it has passed completely through the loom shed. It is necessary to keep the thread taut at all times to avoid so-called thread overrun or a continuing following motion of the thread, which could result in the formation of undesirable loops or tangles in the thread, for example.
Weft thread brakes are conventionally used to avoid the above described formation of loops or tangles. Often, such conventional brakes apply a continuous uniform braking effect to the weft thread. Alternatively, these brakes may be equipped to apply a varying braking effect, that is a braking effect of different strengths, to the thread during the different phases of the weft thread insertion. For example, German Patent Publication (DE-PS) 3,446,567 (May 7, 1986) and corresponding U. S. Pat. No. 4,641,688 (Gehring) (Feb. 10, 1987) discloses a weft thread brake having a stepwise controllable braking effect, for use in shuttleless weaving machines. The disclosed controllable weft thread brake includes two lamellar brake members weft are pressed against each other in the manner of leaf springs. A weft thread pulled from a supply spool is pulled between the two lamellar braking members to apply a braking effect thereto. The spring force of the lamellar braking members pressing against each other defines a base level of the braking effect. The disclosed brake further includes controllable means for controllably increasing the braking effect applied by the lamellar braking members. For example, an electromagnet is disclosed for selectively applying an additional braking or clamping force to the lamellar braking members.
U.S. Pat. No. 4,817,681 (Krumm et al.) (Apr. 4, 1989) discloses another weft thread brake that is controllable in a stepwise manner. The disclosed thread brake uses at least one leaf spring brake element for each weft thread and a common control rail that is activated simultaneously for all leaf springs for pressing the respective weft thread against the respective leaf spring. It has been found that improvements can be made over the weft thread brakes disclosed in U. S. Pat. Nos. 4,817,681 and 4,641,688, especially to achieve an improvement in the adjustability, accuracy, and reproducibility of braking effect settings.
European Pat. Application No. 0,475,892 (Huebner et al.) (Mar. 18, 1992) discloses a thread brake for looms, wherein a weft thread passes between a resilient or flexible brake band and a controllably movable brake shoe body. The brake shoe body can be moved by a servomotor connected to a control and logic circuit and acting through a linkage on the brake shoe body. An inductive sensor is required for positioning the servomotor. The rotational motion of the servomotor shaft is converted into a substantially linear or stroke motion of the brake shoe body through the mechanical linkage. The disclosed thread brake is only capable of releasing or not releasing the braking effect, because a base level braking effect is not provided for. Furthermore, the braking force or braking effect applied by the brake in its closed or engaged position can only be adjusted through a mechanical adjustment device. Once a braking effect adjustment has been selected with the mechanical adjustment device, it cannot be changed during the operation of the loom.
European Pat. Application No. 0,524,429 (Jan. 27, 1993) discloses another thread brake mechanism, especially for use in connection with weft thread insertion in power looms. The disclosed thread brake mechanism includes a shaft, which is rotationally driven by a motor and which has a notch or recess cut out of a portion of its circumference. A flexible tongue, such as a spring tongue, is arranged to press radially against the rotating shaft in an axial region including the cut-out notch in the shaft. The weft thread passes between the spring tongue and the rotating shaft, whereby a cyclically alternating braking effect is applied to the thread. Namely, when the shaft rotates into a position allowing the thread to pass through the cut-out notch, essentially no braking effect is applied and a thread, even including knots, can pass through unhindered. As the shaft rotates further, the cut-out notch turns away and the string is held between the shaft and the spring tongue to apply a braking effect. It is a disadvantage of the disclosed thread brake that only two braking control states exist, namely the thread is allowed to run freely through the cut-out notch without any braking effect or the thread is squeezed between the rotating shaft and the spring tongue to effect braking.